Topic of the Month in December: Creating an optimal glass batch quality

Creating an optimal glass batch quality is not only a matter of mixer performance!

Batch mixers, which belong to the group of mechanical intensive or non-intensive agitators, presently cover a wide capacity range from small laboratory mixers (1litre) up to large industrial ones (5 m³). Types like rotating pan mixers; ribbon mixers; orbiting screw mixers; ploughshare mixers and ring trough mixers are very common in glass industry. Improvement of the agitator tools is still going on in combination with a more efficient way of supplying energy into the batch. Due to an increasing variety of materials to be mixed and an increasing "batch quality” demand, quite a number of mixers have been adapted to more than one area of application. Like many other process equipment, the development of mixers has been typical empirical.

The lack of reliable up-scaling rules is a problem for a process engineer to make the right mixer choice for his specific application. Of course mixer manufacturers undoubtedly are in possession of a large amount of empirical data from their own test runs. However it is regrettable that most mixer manufacturers publish their mixer performance data in a rather confusing way. This often covers their commercial interest, while the mixer performance data is often missing the correct information to enable an objective comparison. Manufacturers mainly present their mixer performance data in terms of "degree of variation”, whatever it means. Reading equipment brochures and magazines it would appear that all mixer types perform at top class with a degree of variation below 0.5% The selection of a mixer remains undoubtedly a difficult matter, while the process engineer will be faced with a wide variety of equipment and relatively little factual data on which to base an informed decision.

Philips in mixing

Philips glass industry has met this diffuse data problem several times during the last 25 years, in trying to make a mixer choice for its glass and none glass batch plants and shows an arbitrary variety of mixers for the same application. Worldwide Philips has some 40 mixers in operation for its powder batching activities with a total bulk amount of about 600,000 tons per year. Some 15 years ago Philips glass industry took the decision to eliminate the less efficient premix groups for micro ingredients (<500 ppm). New types of intensive mixers were able to perform "in line mixing” for micro ingredients down to 100ppm. To investigate this new phenomena and check the factual mixer performance data, a reliable batch homogeneity measurement was needed. The "variance analyses” method had already proven to be a reliable tool in defining the "variation coefficient” (VC) as an index for the batch homogeneity. In case of the micro ingredients "in line mixing”, VC-results showed that the performance of intensive mixers was indeed comparable with the traditional premix process or even better. Consequently, Philips has since eliminated all the premix groups in its batch houses. The variation analysis method proved to be sustainable and is still used for checking the performance of batch mixers and the condition of batch quality.

Defining the VC mixer performance

Although some mixer manufacturers doubt the reliability of defining the batch homogeneity in the mixer itself (preferring sampling at the mixer downstream stretch), Philips does so because the reproducibility has shown good results with the chosen method. In this matter a significant measurable tracer is important for a reliable result. Considering this tracer as a representative for the total batch composition, an amount of 100 ppm is added to the batch. For tracer analysis in the batch, X-ray fluorescence is commonly applied. Also photo spectral analysis and acid soluble methods are in use. For the purpose of analysis, a series of five (20 ppm) samples are taken at different positions in the mixer (a series of ten samples are tested as well, however, the results are not significantly different). Be aware that the samples are not taken from the batch surface layer, which means that a sample spoon with removable cover must be applied (see picture). Sample size is limited at 30ppm for a reliable application of the variance analysis method. The samples are taken out of the frozen condition of the batch in the mixer, by stopping the mixer normally at one-minute intervals.


Sample spoon with cover


Sampling the mixer

Each sample is divided in four sub-samples, resulting in a total of twenty samples to be analyzed. The reason for the sub-sampling is the correct interpretation of the variance analyses by excluding the measuring failure in defining the so called "in between variance” instead of the "total variance”. The total variance outcome always shows a better result and is therefore undoubtedly more often used. The correct interpretation of the variance analyses is checked by the F factor, a statistical check for the variance analysis application.

"F factor” = s² (between) / s² (within) to be > 3.

The formula for the "variance between” is:

s² (between) = {(sum of squares total - sum of squares within)} / {(degrees of freedom total) - (degrees of freedom within)}

The formula for the variation coefficient (homogeneity index):

VC (between) = s (between) / total average * 100%


Example of calculation chart of one series with 5x4 samples

Performance of different mixer types

With the application of the "variance analyses method” Philips has checked the performance of almost all its batch mixers. For each mixer type, a series of 5x4 samples were taken in one-minute intervals up to a total mixing time of 6 minutes. For checking the reproducibility the procedure was repeated twice. Each point in the VC curve is the outcome of 20 analyses, so in total 120 analyses are required to present a VC curve for one mixer. Although the analysis method is expensive, it can of course be compensated by an efficient selection of the tracer material. Obviously costs must always be in coherence with the expected results.

The first series of VC measurements shows the performance of four "non-intensive” mixers, processing dry "lighting” glass batch without cullet, factoring in: the screw orbit mixer, ribbon mixer, ring trough mixer and rotating pan mixer. The batch energy consumption comes to 1 kW / 100kg for the orbit screw mixer and ribbon mixer and 2 kW/100kg for the ring pan mixer and rotating pan mixer.



The rotating pan mixer shows from one minute mixing time onwards the best result. The performance of the orbit screw mixer was additionally checked after a mixing time of 30 minutes, resulting in a VC level of about 35%.

The second series of VC measurements shows the performance of four "intensive mixers” processing dry "lighting” glass batch without cullet, factoring in: the ring trough mixer with intensive rotor, ploughshare mixer with intensive rotor, rotating pan mixer (horizontal type) with intensive rotor and rotating pan mixer (20º inclined type) with intensive rotor. The batch energy consumption comes to 5 kW / 100 kg.



In this series the inclined rotating pan mixer is superior to the other ones. However it is essential and worthwhile to know if the highest created batch homogeneity level is really required in glass industry. It mattered when Philips tried to optimize the productivity of a specific glass plant. In trying to increase the melting capacity of a tube light furnace, Philips performed some investigations in the relation between batch homogeneity and the specific glass-melting rate (SMR). Results did show, that for keeping the 90% furnace productivity, the SMR could only be raised from 60 to 75 (kg/m2.hr), by increasing the batch homogeneity at a level of VC <10. Learning from this phenomena Philips now prefers to use inclined rotating pan mixers for specific applications. In Philips TV Glass business, batch homogeneity requirement are even more stricter down to: VC <5.

What about batch quality!

In the glass industry, batch quality is more than mixer performance, it is the outcome of a chosen compromise between homogeneity, consistency and flow behaviour conditions, knowing that batch consistency and flow behaviour are always in conflict with each other. The maximum attainable batch homogeneity is only related to the mixer performance under the given material grain size conditions. It is common in batch processing that "homogeneity decrease” immediately starts at the moment of mixer discharge and continues downstream by transport manipulation. To restrict the homogeneity decrease is therefore a matter of the right chosen batch consistency level and flow conditions.



Batch consistency can be improved by selecting the batch ingredients in a small grain size, small grain size distribution and batch moisturizing (mainly water or oil). Batch flow behaviour can be improved by selecting the batch ingredients in larger grain size and preferably dry batch. Because of the unavoidable compromise it is important for process engineers that the batch mixing and transport equipment are chosen correctly.

The third series of VC measurements shows the decrease of batch homogeneity in the downstream stretch from mixer to furnace batch charging in coherence with the batch moisture content.



Conclusion

Creating a maximum attainable batch quality on the spot where it is needed isn't only a matter of mixer performance. Batch quality requires an optimal choice of batch consistency to restrict segregation in the downstream batch transport stretch while keeping the optimal batch flow conditions. Batch quality also requires a considered selection of batch transport equipment and "mass flow” silo design for batch storage purpose. Whilst realizing that the required batch homogeneity will be different for each glass application. Personally I think that the application of intensive mixers is a must in the glass industry, because the attainable homogeneity increases favors glass productivity and furnace energy consumption.

Source: OGIS GmbH, glassglobal.com in cooperation with
Philips Lighting Components, department IS&S
Contact: Powder Technology & Project Management, Mr. Fons Rikken 00-31-(0)402788492,
e-mail and with:
Maschinenfabrik Gustav Eirich